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Facile fabrication of tough photocrosslinked polyvinyl alcohol hydrogels with cellulose nanofibrils reinforcement
Accepted Manuscript Facile fabrication of tough photocrosslinked polyvinyl alcohol hydrogels with cellulose nanofibrils reinforcement Junmei Zhang, Tao Liu, Zhenzhen Liu, Qingwen Wang PII:
S0032-3861(19)30348-9
DOI:
https://doi.org/10.1016/j.polymer.2019.04.028
Reference:
JPOL 21411
To appear in:
Polymer
Received Date: 21 February 2019 Revised Date:
10 April 2019
Accepted Date: 11 April 2019
Please cite this article as: Zhang J, Liu T, Liu Z, Wang Q, Facile fabrication of tough photocrosslinked polyvinyl alcohol hydrogels with cellulose nanofibrils reinforcement, Polymer (2019), doi: https:// doi.org/10.1016/j.polymer.2019.04.028. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Facile Fabrication of Tough Photocrosslinked Polyvinyl Alcohol Hydrogels with
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Cellulose Nanofibrils Reinforcement
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Junmei Zhang 1, Tao Liu 1, Zhenzhen Liu * and Qingwen Wang * College of Materials and Energy, South China Agricultural University, 483 Wushan Road,
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Guangzhou 510642, P. R. China
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1
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*Corresponding author: Prof. Zhenzhen Liu; Prof. Qingwen Wang
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Email: [email protected] (Z. Liu); [email protected] (Q. Wang)
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Tel.: +86-20-8528-0319
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These authors contributed equally to this work.
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Graphical abstract:
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Highlights: A tough polyvinyl alcohol hydrogel was fabricated facilely by using photocrosslinked
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polymerization and cellulose nanofibrils (CNF) reinforcement.
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The mechanism of CNF reinforcement was displayed explicitly.
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The fracture compressive stress of this PVA-MA/CNF hydrogel reaches 490 kPa which is higher
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than that of previously reported nanocellulose reinforced PVA hydrogels.
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PVA-MA/CNF hydrogel exhibits excellent cyclic compressive mechanical performance.
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22 Abstract
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A high-strength polyvinyl alcohol (PVA) hydrogel was prepared via a facile photocrosslinked
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polymerization with cellulose nanofibrils (CNF) reinforcement. The mechanism of CNF
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reinforcement was studied systematically. The hydroxyl groups in the surface of CNF generate
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hydrogen bonds with the pendant hydroxyl of PVA chains, which promotes the interaction of already
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chemical crosslinked PVA chains and increases the crystallinity of the resultant PVA-MA/CNF
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hydrogel. By optimizing the CNF incorporation content to the PVA-MA/CNF hydrogel, a
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compressive fracture stress of 490 kPa was obtained, which is higher than that of previously reported
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nanocellulose reinforced PVA hydrogels. And more importantly, this gel exhibits excellent cyclic
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compressive performance. This work provides a novel method to improve the mechanical strength of
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PVA hydrogel which is beneficial for its application in tissue engineering field.
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Keywords:
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cellulose
nanofibrils;
polyvinyl
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alcohol
hydrogel;
high-strength;
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1. Introduction Polyvinyl alcohol (PVA) is one of the most significant, biocompatible synthetic polymers [1,
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2].Because of the hydroxyl group presenting in each repeating unit, PVA exhibits highly hydrophilic
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and hydrogen bonding characteristics [3, 4]. The hydrogen bonding interaction can induce physical
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gelation of PVA chains [2, 5, 6], and the existed hydroxyl groups can also be modified to produce the
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chemical crosslinked hydrogels [4, 7-9]. PVA hydrogels exhibit biocompatible [2, 6, 10, 11],
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biodegradable and resisting protein adsorption and cell adhesion [12, 13], which have been utilized
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in several biomedical [13, 14] and pharmaceutical application [8, 15]. However, the urgently need of
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high mechanical strength hydrogel for suffering large stress in use, such as cartilage, tendon,
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ligament repair and other tissue engineering scaffolds [13, 16, 17], promoting to develop high
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strength and toughness PVA hydrogels.
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Many efforts have been devoted to improve the mechanical strength of PVA hydrogels. The
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freeze-thaw method is experimentally straightforward and not require any chemical crosslinking
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agent, and the physical crosslinked PVA hydrogel prepared by this method [2, 18-20] shows higher
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mechanical strength. However, the time-consuming freezing-thawing process and high energy
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consumption go against the low-carbon economy. Alternatively, exploration of chemical crosslinker
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to connect PVA chains has been found to be an efficient and feasible way to fabricate PVA
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hydrogels with better properties, such as glutaraldehyde [9, 21, 22] and borax [23-25] reinforced
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PVA hydrogels.
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Cellulose nanofibrils (CNF), displaying excellent mechanical properties, high surface areas,
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readily biocompatibility and biodegradability [5, 26, 27], have been incorporated into the numerous
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polymer matrix to reinforce the hydrogels’ [18, 19, 27-29] or aerogels’ [5, 9, 26, 29] mechanical
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strength. CNF can be synthesized using various methods including mechanical shearing, 2,2,6,6-
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tetramethylpiperidine-1-oxyl (TEMPO) oxidation, enzymatic hydrolysis of macroscopic cellulose
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followed by high-pressure homogenization or ultrasonication [30, 31].The utilization of CNF in the
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glutaraldehyde or borax crosslinked PVA hydrogels [28, 29], or CNF reinforced cyclic freeze-thaw
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physical crosslinked PVA hydrogels [18, 19]. Although the mechanical property of the composited
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PVA hydrogels were improved, the much higher mechanical strength of PVA hydrogels are urgently
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needed to accommodate the desired specific tissue engineering applications [32-34].
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Moreover, photocrosslinked polymerization is a facile, biocompatible approach, which has been
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used to construct 3D cell culture matrix or tissue scaffold. The gelation could occur in minutes under
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light irradiation with a little amount photoinitiator [35, 36].
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Inspired by the previous work, we fabricated a tough PVA hydrogel by exploiting the advantage of
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photocrosslinking method and the reinforcement effect of CNF. As shown in Scheme 1, a
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photoinitiated functional methacrylate group was firstly grafted into PVA polymer upon reacting
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with the pendant hydroxyl groups of PVA chains to generate PVA-MA, subsequently the mixture
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solution of PVA-MA, CNF, photoinitiator I2959 were irradiated under UV light, facilely construct a
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higher mechanical PVA hydrogel. The tough mechanism is based on the hypothesis that the abundant
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hydroxyl groups in the surface of the crystalline CNF can increase the interaction of already
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photoinduced chemical crosslinked PVA chains via hydrogen bonding, which can increase the
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crystallinity of PVA hydrogels. The objective of this work is to prepare higher mechanical PVA
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Scheme 1 Schematic illustration of facile, one-pot fabrication of PVA-MA/CNF hydrogel with high compressive stress and toughness.
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adjusting the incorporation content of CNF in hydrogel formulation, a high mechanical strength of
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490 kPa (fracture compressive stress) is obtained which is higher than that of previously reported
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nanocellulose reinforced PVA hydrogels. And the synthetic optimized PVA hydrogel also exhibits
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excellent cyclic compressive performance.
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2. Experimental
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2.1. Materials
Polyvinyl alcohol (average MW = 94899 g/mol) was purchased from Sinopharm Chemical Reagent
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Co.,
Methacrylate glycidyl
ether (GMA,
97%) was
bought
from
Macklin.
4-
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dimethylaminopyridine (DMAP, 99%) was got from TCI. Dimethyl sulfoxide (DMSO, 99.7%) and
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acetone (99.7%) were purchased from Tianjin Damao Chemical Reagent factory. Cellulose powder
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was bought from Aladdin. 2,2,6,6-tetramethyl piperidine oxide (TEMPO, 98.5%), sodium
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hypochlorite (NaClO, 99.7%), sodium hydrate (NaOH, 97%), absolute ethyl alcohol and 2-Hydroxy-
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4'-(2-hydroxyethoxy)-2-methylpropiophenone (I2959, 98%) was purchased from Energy Chemical.
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Monometallic sodium orthophosphate (NaH2PO4, 99.7%) and disodium hydrogen phosphate
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(Na2HPO4, 99.7%) were got from Acros. Sodium chloride (NaCl, 99.7%) and potassium chloride
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(KCl, 99.7%) were purchased from Shanghai Rich Joint Chemical Reagents Co., Ltd. Hydrochloric
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acid (HCl, 99.8%) was bought from Guangzhou Chemical Reagent Company.
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2.2. Preparation of CNF
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CNF was prepared from cellulose powder via the preparation technique of TEMPO-mediated
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oxidation and high pressure homogenization treatment. 2.5 g cellulose powder were mixed into 250
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ml water containing 0.04 g TEMPO (0.25 mmol) and 0.25 g sodium bromide (2.5 mmol). Then
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adding NaClO solution (6%wt) to the above solution and adjusted to pH = 10 by the addition of
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NaOH solution (5%wt). Next, the TEMPO-oxidized cellulose was centrifuged and washed until the
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PH was close to 7, then the centrifuged TEMPO-oxidized cellulose was handled 20 cycles under 900
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(MWCO = 7000 D) for 3 days. And the purified CNF was gained after freeze-drying by a vacuum
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freezing dryer (Scientz-10 N). The carboxylate content of the TEMPO-oxidized cellulose was
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determined as 2 mmol/g using an electric conductivity titration method.
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2.3. Synthesis of methacrylate modified Polyvinyl Alcohol (PVA-MA)
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PVA-MA was synthesized (see Fig. S1) by the reaction of the hydroxyl groups on the pristine
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PVA with the epoxy groups on the methacrylate glycidyl ether under the dimethyl sulfoxide as
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solvent. Specifically, the preparation of PVA-MA macromer was carried out through the following
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methods. 10 g PVA and 0.22 g DMAP were mixed with 100 ml DMSO. Accordingly, the whole
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system was heated at 60 °C under N2 atmosphere until PVA and DMAP were completely dissolved.
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After that, 0.64 g methacrylate glycidyl ether was dropped into the above system and reacted 12 h at
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60 °C under N2 atmosphere. After the reaction completed, the reaction mixture was added into 400
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ml acetone to precipitate, and the crude product was collected by filtrating, then dissolved in 100 ml
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deionized water and dialysed (MWCO = 7000 D) for 4 days. The purified PVA-MA was obtained
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after freeze-drying.
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2.4. Fabrication of PVA-MA/CNF hydrogel
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500 mg PVA-MA was dissolved in 5 ml PBS buffer at 90 °C. After the PVA-MA solution was
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cooled to room temperature and sonicated for 20 min to remove bubbles, the photoinitiator I2959
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(0.1%wt based on the weight of PVA-MA) was added into the solution, and 10%wt hydrogel
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precursor solution was prepared. Then the hydrogel precursor solution was injected into a cylindrical
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mold and subsequently irradiated on 365 nm (25 mw/cm2) for 6 min to prepare photocrosslinked
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PVA-MA hydrogel, which was used as the control group. Referring to the fabrication of PVA-MA
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hydrogel, the PVA-MA/CNF hydrogel was prepared by the following method: PVA-MA was firstly
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dissolved in PBS buffer and then I2959 and specific CNF were added into the above solution to
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prepare hydrogel precursor solution (10%wt PVA-MA, 0.1%wt I2959 (w/w = I2959/PVA-MA)).
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The weight percentage of CNF was determined based on the weight of PVA-MA, and coded as
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PVA-MA/CNF 0.5 (0.5%wt CNF), PVA-MA/CNF 1 (1%wt CNF), PVA-MA/CNF 2 (2%wt CNF),
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PVA-MA/CNF 4 (4%wt CNF), respectively. The detailed formulation of the PVA-MA/CNF
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hydrogels was shown in Table 1.
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mH2O (ml)
mCNF (mg)
PVA-MA hydrogel
5
0
PVA-MA/CNF 0.5
5
2.5
PVA-MA/CNF 1
5
5
PVA-MA/CNF 2
5
PVA-MA/CNF 4
5
mPVA-MA (mg) 500
500
500
10
500
20
500
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Table 1. Compositions of PVA-MA/CNF hydrogels
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2.5. Characterization
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2.5.1. Transmission electron microscopy (TEM) characterization
The Transmission electron microscope (TEM) images of the CNF suspension and PVA-MA/CNF
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mixed solution were observed using a FEI Talos F200S (America) with an accelerating voltage of
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200 kV. A few drops of the sample solution (0.1%wt) was placed onto copper grids covered with
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holey carbon support films, excess liquid was blotted with filter paper. Then a small droplet of uranyl
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acetate (2%wt) was added, and the liquid in excess was removed. The stained samples were obtained
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after dried at room temperature and can be further analyzed.
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2.5.2. Dynamic light scattering (DLS) characterization
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The particle size and surface charge of the prepared CNF was determined by DLS experiments
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using Zetasizer Nano ZSE. Before measurements, the samples should be configured into 0.001%wt
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suspensions and dispersed under sonication.
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2.5.3. Nuclear magnetic resonance (NMR) characterization
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H NMR experiment was carried out by using a Bruker NMR spectrometer (600MHz, Germany),
and the dimethyl sulfoxide (DMSO)-d6 as the solvent.
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2.5.4. Fourier transform infrared (FTIR) characterization Fourier Transform Infrared (FTIR) spectrometer (PerkinElmer, USA) was utilized to confirm the
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successful synthesis of PVA-MA, the occurrence of photopolymerization of PVA-MA and PVA-
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MA/CNF and the CNF interaction with PVA chains. The test samples were prepared by mixing with
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KBr pellets by a bead machine.
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2.5.5. Compression stress-strain tests of hydrogels
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Compression stress-strain tests were carried out with a universal mechanical testing machine
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(Shimadzu, AGS-X, JP). Cylindrical hydrogel samples with a height of 5-6 mm and a diameter of 12
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mm were used for compression tests. Prior to each test, a preload of 0.1 N was used to stabilize the
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cylindrical hydrogel samples. Each compression test was carried out at a rate of 2 mm/min at room
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temperature except for the continuous cyclic compression test with a strain of 50% at a rate of 4
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mm/min at room temperature.
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2.5.6. X-ray Diffraction Spectroscopy (XRD) characterization
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The X-ray diffraction (XRD) spectroscopy of the samples were obtained by using the X-ray
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diffractometer (Beijing general instrument co., Ltd., XD-2X/M4600, CHN) in the range of 10-30° in
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steps of 2°. All the samples were freeze-dried and grinded into powder before used.
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2.5.7. Scanning electron microscopy (SEM) characterization
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The fracture surfaces of the hydrogel samples were observed by scanning electron microscope
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(SEM, EVO MA15, Zeiss, Germany). Before testing, all the samples were fractured with liquid
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nitrogen and freeze-dried immediately, and sputter-coated with a thin Au layer.
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3. Results and discussion
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3.1. Characterization of CNF and PVA-MA
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As shown in Fig. S2a, CNF dispersed well in water with the length ranging from one to several
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microns, which promoted the favorable dispersion of CNF into PVA-MA solution (Fig. S2b).
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Moreover, the insets of Fig. S2a and Fig. S2b depicted that the tyndall effect emerged by irradiating
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was dispersed well in aqueous solution or PVA-MA solution. The average particle size (267nm, Fig.
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S3) and surface with negative charge (Fig. S4) of CNF were tested by DLS. The functional carboxyl
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groups in the surface of CNF can effectively inhibit the aggregation of CNF, which can promote the
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sufficient interaction of CNF with PVA matrix. PVA-MA was firstly characterized by 1H NMR (Fig.
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1). Compared to the spectrum of pristine PVA, new peaks at 6.0 and 5.6 ppm appeared which
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ascribed to the characteristic double bond of methacrylate group, demonstrating the methacrylate
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group was successfully grafted onto the pendant hydroxyl groups of PVA. And the degree of
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substitution (DS) of PVA-MA was determined as 4% by calculating the integration of characteristic
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peak of methacrylate at 6.0, 5.6 ppm and the backbone of PVA at 3.8 ppm. FTIR was also used to
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confirm the effectiveness of methacrylate functionalization of PVA-MA (Fig. 2a). The increased
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Fig. 1 1H NMR spectra of PVA-MA and the pristine PVA ((DMSO)-d6).
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intensity of the bands at 1720 cm-1 and 1640 cm-1 in PVA-MA compared to the pristine PVA was
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attributed to the C=O and C=C stretching vibration, indicating the methacrylate groups were
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successfully grafted to the PVA chains.
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3.2. FTIR analysis of PVA-MA/CNF hydrogel The effectiveness of photocrosslinking reaction and CNF incorporation were firstly verified by
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FTIR. As shown in Fig. 2a, the evident band in PVA-MA hydrogel and PVA-MA/CNF hydrogel
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sample at 1640 cm-1 (νC=C) disappeared indicating the successful occurrence of photoinitiated
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gelation. And the stretching vibration of hydroxyl group at 3367 cm-1 shifted to 3350 cm-1 because of
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the incorporation of CNF in PVA-MA/CNF hydrogel sample, indicating the occurrence of hydrogen
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bond of CNF to the PVA chains. Furthermore, the transparency of PVA-MA/CNF hydrogel
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decreased evidently compared with that of PVA-MA hydrogel under the same thickness (Fig. 2b, c).
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This phenomenon is not only attributed to the light scattering of CNF particles, the high aspect ratio
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of CNF promoting the entanglement of polymer chains and the hydrogen bond between CNF and
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PVA chains induced the increasement of physical junctions of hydrogel network also affected the
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transparency. This behavior strongly demonstrates the efficient interaction of CNF to the PVA
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matrix.
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Fig. 2 (a) FTIR spectra of the pristine PVA, PVA-MA, PVA-MA hydrogel, PVA-MA/CNF1 hydrogel and CNF; Photographs of PVA-MA and PVA-MA/CNF1 hydrogel in top side (b) and vertical side (c).
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3.3. Mechanical performance of the PVA-MA/CNF hydrogel The compressive strength of hydrogel is an important property for load-bearing application in
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cartilage repair or other tissue engineering scaffolds [16, 17, 37, 38]. Fig. 3a clearly shows that the
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incorporation of CNF obviously improves the compressive stress of PVA-MA hydrogels. And the
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PVA-MA/CNF 1 hydrogel exhibits the highest compressive fracture stress (490 kPa) with the
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fracture strain of 73%, which is higher than that of previously reported PVA hydrogels (Table 2, 3).
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Remarkably, the PVA-MA/CNF 1 hydrogel was foldable and quickly recovered to its original state
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without any rupture after compression, while the PVA-MA hydrogel was easy to rupture into small
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pieces under the same compressive condition, demonstrating the excellent flexibility and elasticity of
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CNF reinforced PVA-MA hydrogels (Fig. 3b, Supplementary Movie 1).
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Table 2. The fracture stress and strain results of PVA-MA/CNF hydrogels and PVA-MA hydrogel Samples
fracture
PVA-MA
PVA-MA/CNF 0.5
PVA-MA/CNF 1
PVA-MA/CNF 2
PVA-MA/CNF 4
hydrogel
hydrogel
hydrogel
hydrogel
hydrogel
140
315
490
358
273
73
70
74
fracture
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Fig. 3 (a) Compressive stress-strain curves of PVA-MA hydrogel and PVA-MA/CNF hydrogel; (b) Photographs of the compression behavior of PVA-MA hydrogel and PVA-MA/CNF1 hydrogel under
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Except displaying a significant improvement of stress at fracture, the PVA-MA/CNF hydrogels
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also possess high toughness which confirmed by the loading-unloading compressive tests. These
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cycle curves were obtained immediately after one loading-unloading cycle. As shown in Fig. 4a, the
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stress is increasing with the applied strain from 40%-70%, and it can return to the original state after
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unloading for each cycle.
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The value of corresponding stress at specific strain is also consistent with that in fracture
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compression test. Specifically, no breakage or even visible cracking was observed for PVA-
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MA/CNF 1 hydrogel after 300 successive loading-unloading compressive cycles at a set strain of
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50%, and no evident decrease of stress was shown in the first 50 cycles (Fig. 4b). Even at a set strain
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of 60%, this gel still could resist more than 100 successive loading-unloading compressive cycles,
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and the value of stress remained about the same in the first 10 cycles (Fig. 4c). This excellent
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recovery property of PVA-MA/CNF hydrogel can be explained by the efficient energy dissipating
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Fig. 4 Toughness mechanical property of PVA-MA/CNF1 hydrogel. (a) Compressive stress-strain curves with a varying maximum compression strain, the inset is amplification figure of the initial part; (b) Compressive stress-strain curves at 50% strain under loading-unloading cycles, the inset is curves for 1st-50th cycles; (c) Compressive 12 stress-strain curves at 60% strain under loadingunloading cycles, the inset is curves for 1st-10th cycles.
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mechanism. During the process of PVA-MA/CNF hydrogel deformation, the reversible physical
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interactions between PVA- CNF chains or PVA, CNF chains can dissipate energy effectively,
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resulting to the high mechanical strength and toughness of hydrogel.
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3.4. Crystallinity of PVA-MA/CNF hydrogel Diffraction patterns of PVA-MA, PVA-MA/CNF hydrogels are shown in Fig. 5. The
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photocrosslinked PVA-MA hydrogel showed a diffraction peak at 2θ = 19.4o, being assigned to the
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orthorhombic lattice structure of semicrystalline PVA [11]. And a sharp high peak at 2θ = 22.6o and
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two overlapped weaker diffraction peaks at 2θ = 14.8o, 16.6o appeared in CNF sample [39], which
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was consistent with the diffraction peak of crystallographic form of cellulose I. With the
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incorporation of CNF to PVA matrix from 0%wt-4%wt, the crystallinity of PVA-MA/CNF hydrogel
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increases from 0%-1%, and shows highest for PVA-MA/CNF 1 hydrogel, then decreases from 1%-
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4%, and the CNF diffraction peak appeared for PVA-MA/CNF 4 hydrogel. The above result
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demonstrates the CNF incorporation content has an appropriate value for increasing the crystallinity
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of the composited hydrogels. The more CNF in PVA matrix is prone to generate hydrogen bond
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between CNF-CNF, and less physical interaction between CNF-PVA chains, sinificantly disturb the
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13 Fig. 5 The XRD patterns of PVA-MA hydrogel, PVA-MA/CNF hydrogel and CNF.
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crystallization of PVA-MA hydrogels, and exhibiting lower mechanical strength which is consistant
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with the result of mechanical test.
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3.5. Morphological analysis of PVA-MA/CNF hydrogel In order to investigate the 3D network structure of PVA-MA/CNF hydrogel, the scanning electron
244
microscopy (SEM) was used to analyze the hydrogel samples which were treated by freeze-drying
245
technique. And the photocrosslinked PVA-MA hydrogel was as a control sample (Fig. 6a1, a2). As
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shown in Fig. 6b-e, no significant aggregation of CNF in PVA-MA/CNF hydrogel samples is
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detected, demonstrating the CNF dispersed well in PVA matrix for composite hydrogels. Among the
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different PVA-MA/CNF hydrogel samples, PVA-MA/CNF 2 (Fig. 6d1, d2) and PVA-MA/CNF 4
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(Fig. 6e1, e2) clearly show the disorganized structure because of the more CNF-CNF interaction, and
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the PVA-MA/CNF 1 (Fig. 6c1, c2) shows the best uniform interconnected porous structure. The
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homogeneous structure of PVA-MA/CNF 1 with tighter networks was believed to be responsible for
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the excellent mechanical properties, which corresponds well with the result of FTIR, compressive
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mechanical test and XRD. Furthermore, the interconnected holes are beneficial for the biological
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Fig. 6 Scanning electron microscopy images of PVA-MA hydrogel (a1, a2), PVA-MA/CNF0.5 hydrogel (b1, b2), PVA-MA/CNF1 hydrogel (c1, c2), PVA-MA/CNF2 hydrogel (d1, d2), and PVA-MA/CNF4 hydrogel (e1, e2), the scale bar is 10 µm (top) and 3 µm (bottom).
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property of hydrogels, which promote cells grow and migrate or make nutrients or growth factors
255
into the material inside.
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4. Conclusions 14
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with CNF reinforcement. Except avoiding using the small molecule chemical crosslinking agent and
259
time-consuming freeze-thaw process, a much higher mechanical strength PVA hydrogel was
260
successfully prepared by optimizing the CNF loading content, and the compressive fracture stress
261
reaches 490 kPa which is higher than that of previously reported nanocellulose reinforced PVA
262
hydrogels. Specifically, this kind of PVA-MA/CNF hydrogels exhibit excellent toughness, which can
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resist hundreds of successive loading-unloading compressions. And the mechanism of CNF
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reinforcement was systematically studied. The biocompatibility of the synthetic tough PVA hydrogel
265
and its degradation products will be done in future work. This fabricated tough PVA hydrogels
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greatly advance their potential to be applied in tissue engineering scaffold and biomedical field.
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Acknowledgements
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This work was supported by National Natural Science Foundation of China [31741022]; Natural
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Science Foundation of Guangdong Province [2018A030310146].
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Table 3. The fracture stress and strain results of different kinds of PVA hydrogels PVA weight
filler / filler weight
time (h)
weight (g/mol)
percent (%wt)
percent (%wt)
Compressive fracture stress (kPa) and fracture strain (%)
photocrosslinking
0.1
94,899
10 (PVA-MA)
CNF, 1
490, 73
[19]
1 freeze-thaw cycle
14
89,000-98,000
4
CNF/LCNF, 2
≈138, 60
[18]
3-5 freeze-thaw cycles
≈104
89,000-98,000
10
CNC, 1
37, 50
[20]
5 freeze-thaw cycles
50
88,000
10
graphite oxide, 0.025
102, 53
2
N/A
1.5
Acrylamide, 8
16, 75
chemical crosslinking with borax chemical crosslinking with borax
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0.08
89,000-98,000
4
CNC, 1
60, 53
1
146,000-186,000
2
CNC/CNF, 1
71, 95
5
CNC, 40
33, N/A
6
TE D
[25]
chemical crosslinking
61,000
EP
[27]
acrylamide
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[22]
double network with
SC
this work
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references
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ACCEPTED MANUSCRIPT Highlights: A tough polyvinyl alcohol hydrogel was fabricated facilely by using photocrosslinked polymerization and cellulose nanofibrils (CNF) reinforcement.
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The mechanism of CNF reinforcement was displayed explicitly. The fracture compressive stress of this PVA-MA/CNF hydrogel reaches 490 kPa which is higher than that of previously reported nanocellulose reinforced PVA
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hydrogels.
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PVA-MA/CNF hydrogel exhibits excellent cyclic compressive mechanical
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performance.
S0032-3861(19)30348-9
DOI:
https://doi.org/10.1016/j.polymer.2019.04.028
Reference:
JPOL 21411
To appear in:
Polymer
Received Date: 21 February 2019 Revised Date:
10 April 2019
Accepted Date: 11 April 2019
Please cite this article as: Zhang J, Liu T, Liu Z, Wang Q, Facile fabrication of tough photocrosslinked polyvinyl alcohol hydrogels with cellulose nanofibrils reinforcement, Polymer (2019), doi: https:// doi.org/10.1016/j.polymer.2019.04.028. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT 1
Facile Fabrication of Tough Photocrosslinked Polyvinyl Alcohol Hydrogels with
2
Cellulose Nanofibrils Reinforcement
3
Junmei Zhang 1, Tao Liu 1, Zhenzhen Liu * and Qingwen Wang * College of Materials and Energy, South China Agricultural University, 483 Wushan Road,
5
Guangzhou 510642, P. R. China
6
1
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*Corresponding author: Prof. Zhenzhen Liu; Prof. Qingwen Wang
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Email: [email protected] (Z. Liu); [email protected] (Q. Wang)
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Tel.: +86-20-8528-0319
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These authors contributed equally to this work.
10 11
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Graphical abstract:
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13 14
1
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Highlights: A tough polyvinyl alcohol hydrogel was fabricated facilely by using photocrosslinked
17
polymerization and cellulose nanofibrils (CNF) reinforcement.
18
The mechanism of CNF reinforcement was displayed explicitly.
19
The fracture compressive stress of this PVA-MA/CNF hydrogel reaches 490 kPa which is higher
20
than that of previously reported nanocellulose reinforced PVA hydrogels.
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PVA-MA/CNF hydrogel exhibits excellent cyclic compressive mechanical performance.
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16
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22 Abstract
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A high-strength polyvinyl alcohol (PVA) hydrogel was prepared via a facile photocrosslinked
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polymerization with cellulose nanofibrils (CNF) reinforcement. The mechanism of CNF
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reinforcement was studied systematically. The hydroxyl groups in the surface of CNF generate
27
hydrogen bonds with the pendant hydroxyl of PVA chains, which promotes the interaction of already
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chemical crosslinked PVA chains and increases the crystallinity of the resultant PVA-MA/CNF
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hydrogel. By optimizing the CNF incorporation content to the PVA-MA/CNF hydrogel, a
30
compressive fracture stress of 490 kPa was obtained, which is higher than that of previously reported
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nanocellulose reinforced PVA hydrogels. And more importantly, this gel exhibits excellent cyclic
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compressive performance. This work provides a novel method to improve the mechanical strength of
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PVA hydrogel which is beneficial for its application in tissue engineering field.
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Keywords:
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cellulose
nanofibrils;
polyvinyl
2
alcohol
hydrogel;
high-strength;
ACCEPTED MANUSCRIPT 35
1. Introduction Polyvinyl alcohol (PVA) is one of the most significant, biocompatible synthetic polymers [1,
37
2].Because of the hydroxyl group presenting in each repeating unit, PVA exhibits highly hydrophilic
38
and hydrogen bonding characteristics [3, 4]. The hydrogen bonding interaction can induce physical
39
gelation of PVA chains [2, 5, 6], and the existed hydroxyl groups can also be modified to produce the
40
chemical crosslinked hydrogels [4, 7-9]. PVA hydrogels exhibit biocompatible [2, 6, 10, 11],
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biodegradable and resisting protein adsorption and cell adhesion [12, 13], which have been utilized
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in several biomedical [13, 14] and pharmaceutical application [8, 15]. However, the urgently need of
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high mechanical strength hydrogel for suffering large stress in use, such as cartilage, tendon,
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ligament repair and other tissue engineering scaffolds [13, 16, 17], promoting to develop high
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strength and toughness PVA hydrogels.
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Many efforts have been devoted to improve the mechanical strength of PVA hydrogels. The
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freeze-thaw method is experimentally straightforward and not require any chemical crosslinking
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agent, and the physical crosslinked PVA hydrogel prepared by this method [2, 18-20] shows higher
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mechanical strength. However, the time-consuming freezing-thawing process and high energy
50
consumption go against the low-carbon economy. Alternatively, exploration of chemical crosslinker
51
to connect PVA chains has been found to be an efficient and feasible way to fabricate PVA
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hydrogels with better properties, such as glutaraldehyde [9, 21, 22] and borax [23-25] reinforced
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PVA hydrogels.
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Cellulose nanofibrils (CNF), displaying excellent mechanical properties, high surface areas,
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readily biocompatibility and biodegradability [5, 26, 27], have been incorporated into the numerous
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polymer matrix to reinforce the hydrogels’ [18, 19, 27-29] or aerogels’ [5, 9, 26, 29] mechanical
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strength. CNF can be synthesized using various methods including mechanical shearing, 2,2,6,6-
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tetramethylpiperidine-1-oxyl (TEMPO) oxidation, enzymatic hydrolysis of macroscopic cellulose
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followed by high-pressure homogenization or ultrasonication [30, 31].The utilization of CNF in the
3
ACCEPTED MANUSCRIPT preparation of enhanced PVA hydrogels has also been explored, such as CNF reinforced
61
glutaraldehyde or borax crosslinked PVA hydrogels [28, 29], or CNF reinforced cyclic freeze-thaw
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physical crosslinked PVA hydrogels [18, 19]. Although the mechanical property of the composited
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PVA hydrogels were improved, the much higher mechanical strength of PVA hydrogels are urgently
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needed to accommodate the desired specific tissue engineering applications [32-34].
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Moreover, photocrosslinked polymerization is a facile, biocompatible approach, which has been
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used to construct 3D cell culture matrix or tissue scaffold. The gelation could occur in minutes under
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light irradiation with a little amount photoinitiator [35, 36].
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Inspired by the previous work, we fabricated a tough PVA hydrogel by exploiting the advantage of
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photocrosslinking method and the reinforcement effect of CNF. As shown in Scheme 1, a
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photoinitiated functional methacrylate group was firstly grafted into PVA polymer upon reacting
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with the pendant hydroxyl groups of PVA chains to generate PVA-MA, subsequently the mixture
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solution of PVA-MA, CNF, photoinitiator I2959 were irradiated under UV light, facilely construct a
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higher mechanical PVA hydrogel. The tough mechanism is based on the hypothesis that the abundant
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hydroxyl groups in the surface of the crystalline CNF can increase the interaction of already
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photoinduced chemical crosslinked PVA chains via hydrogen bonding, which can increase the
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crystallinity of PVA hydrogels. The objective of this work is to prepare higher mechanical PVA
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Scheme 1 Schematic illustration of facile, one-pot fabrication of PVA-MA/CNF hydrogel with high compressive stress and toughness.
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ACCEPTED MANUSCRIPT hydrogel using a facile, biocompatible approach, and demonstrate the reinforced mechanism. By
78
adjusting the incorporation content of CNF in hydrogel formulation, a high mechanical strength of
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490 kPa (fracture compressive stress) is obtained which is higher than that of previously reported
80
nanocellulose reinforced PVA hydrogels. And the synthetic optimized PVA hydrogel also exhibits
81
excellent cyclic compressive performance.
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2. Experimental
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2.1. Materials
Polyvinyl alcohol (average MW = 94899 g/mol) was purchased from Sinopharm Chemical Reagent
85
Co.,
Methacrylate glycidyl
ether (GMA,
97%) was
bought
from
Macklin.
4-
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dimethylaminopyridine (DMAP, 99%) was got from TCI. Dimethyl sulfoxide (DMSO, 99.7%) and
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acetone (99.7%) were purchased from Tianjin Damao Chemical Reagent factory. Cellulose powder
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was bought from Aladdin. 2,2,6,6-tetramethyl piperidine oxide (TEMPO, 98.5%), sodium
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hypochlorite (NaClO, 99.7%), sodium hydrate (NaOH, 97%), absolute ethyl alcohol and 2-Hydroxy-
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4'-(2-hydroxyethoxy)-2-methylpropiophenone (I2959, 98%) was purchased from Energy Chemical.
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Monometallic sodium orthophosphate (NaH2PO4, 99.7%) and disodium hydrogen phosphate
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(Na2HPO4, 99.7%) were got from Acros. Sodium chloride (NaCl, 99.7%) and potassium chloride
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(KCl, 99.7%) were purchased from Shanghai Rich Joint Chemical Reagents Co., Ltd. Hydrochloric
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acid (HCl, 99.8%) was bought from Guangzhou Chemical Reagent Company.
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2.2. Preparation of CNF
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CNF was prepared from cellulose powder via the preparation technique of TEMPO-mediated
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oxidation and high pressure homogenization treatment. 2.5 g cellulose powder were mixed into 250
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ml water containing 0.04 g TEMPO (0.25 mmol) and 0.25 g sodium bromide (2.5 mmol). Then
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adding NaClO solution (6%wt) to the above solution and adjusted to pH = 10 by the addition of
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NaOH solution (5%wt). Next, the TEMPO-oxidized cellulose was centrifuged and washed until the
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PH was close to 7, then the centrifuged TEMPO-oxidized cellulose was handled 20 cycles under 900
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ACCEPTED MANUSCRIPT bar with high pressure homogenization. The obtained raw CNF was put into the dialysis bag
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(MWCO = 7000 D) for 3 days. And the purified CNF was gained after freeze-drying by a vacuum
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freezing dryer (Scientz-10 N). The carboxylate content of the TEMPO-oxidized cellulose was
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determined as 2 mmol/g using an electric conductivity titration method.
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2.3. Synthesis of methacrylate modified Polyvinyl Alcohol (PVA-MA)
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PVA-MA was synthesized (see Fig. S1) by the reaction of the hydroxyl groups on the pristine
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PVA with the epoxy groups on the methacrylate glycidyl ether under the dimethyl sulfoxide as
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solvent. Specifically, the preparation of PVA-MA macromer was carried out through the following
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methods. 10 g PVA and 0.22 g DMAP were mixed with 100 ml DMSO. Accordingly, the whole
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system was heated at 60 °C under N2 atmosphere until PVA and DMAP were completely dissolved.
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After that, 0.64 g methacrylate glycidyl ether was dropped into the above system and reacted 12 h at
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60 °C under N2 atmosphere. After the reaction completed, the reaction mixture was added into 400
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ml acetone to precipitate, and the crude product was collected by filtrating, then dissolved in 100 ml
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deionized water and dialysed (MWCO = 7000 D) for 4 days. The purified PVA-MA was obtained
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after freeze-drying.
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2.4. Fabrication of PVA-MA/CNF hydrogel
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500 mg PVA-MA was dissolved in 5 ml PBS buffer at 90 °C. After the PVA-MA solution was
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cooled to room temperature and sonicated for 20 min to remove bubbles, the photoinitiator I2959
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(0.1%wt based on the weight of PVA-MA) was added into the solution, and 10%wt hydrogel
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precursor solution was prepared. Then the hydrogel precursor solution was injected into a cylindrical
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mold and subsequently irradiated on 365 nm (25 mw/cm2) for 6 min to prepare photocrosslinked
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PVA-MA hydrogel, which was used as the control group. Referring to the fabrication of PVA-MA
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hydrogel, the PVA-MA/CNF hydrogel was prepared by the following method: PVA-MA was firstly
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dissolved in PBS buffer and then I2959 and specific CNF were added into the above solution to
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prepare hydrogel precursor solution (10%wt PVA-MA, 0.1%wt I2959 (w/w = I2959/PVA-MA)).
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The weight percentage of CNF was determined based on the weight of PVA-MA, and coded as
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PVA-MA/CNF 0.5 (0.5%wt CNF), PVA-MA/CNF 1 (1%wt CNF), PVA-MA/CNF 2 (2%wt CNF),
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PVA-MA/CNF 4 (4%wt CNF), respectively. The detailed formulation of the PVA-MA/CNF
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hydrogels was shown in Table 1.
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mH2O (ml)
mCNF (mg)
PVA-MA hydrogel
5
0
PVA-MA/CNF 0.5
5
2.5
PVA-MA/CNF 1
5
5
PVA-MA/CNF 2
5
PVA-MA/CNF 4
5
mPVA-MA (mg) 500
500
500
10
500
20
500
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Table 1. Compositions of PVA-MA/CNF hydrogels
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2.5. Characterization
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2.5.1. Transmission electron microscopy (TEM) characterization
The Transmission electron microscope (TEM) images of the CNF suspension and PVA-MA/CNF
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mixed solution were observed using a FEI Talos F200S (America) with an accelerating voltage of
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200 kV. A few drops of the sample solution (0.1%wt) was placed onto copper grids covered with
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holey carbon support films, excess liquid was blotted with filter paper. Then a small droplet of uranyl
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acetate (2%wt) was added, and the liquid in excess was removed. The stained samples were obtained
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after dried at room temperature and can be further analyzed.
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2.5.2. Dynamic light scattering (DLS) characterization
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The particle size and surface charge of the prepared CNF was determined by DLS experiments
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using Zetasizer Nano ZSE. Before measurements, the samples should be configured into 0.001%wt
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suspensions and dispersed under sonication.
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2.5.3. Nuclear magnetic resonance (NMR) characterization
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H NMR experiment was carried out by using a Bruker NMR spectrometer (600MHz, Germany),
and the dimethyl sulfoxide (DMSO)-d6 as the solvent.
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2.5.4. Fourier transform infrared (FTIR) characterization Fourier Transform Infrared (FTIR) spectrometer (PerkinElmer, USA) was utilized to confirm the
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successful synthesis of PVA-MA, the occurrence of photopolymerization of PVA-MA and PVA-
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MA/CNF and the CNF interaction with PVA chains. The test samples were prepared by mixing with
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KBr pellets by a bead machine.
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2.5.5. Compression stress-strain tests of hydrogels
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Compression stress-strain tests were carried out with a universal mechanical testing machine
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(Shimadzu, AGS-X, JP). Cylindrical hydrogel samples with a height of 5-6 mm and a diameter of 12
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mm were used for compression tests. Prior to each test, a preload of 0.1 N was used to stabilize the
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cylindrical hydrogel samples. Each compression test was carried out at a rate of 2 mm/min at room
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temperature except for the continuous cyclic compression test with a strain of 50% at a rate of 4
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mm/min at room temperature.
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2.5.6. X-ray Diffraction Spectroscopy (XRD) characterization
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The X-ray diffraction (XRD) spectroscopy of the samples were obtained by using the X-ray
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diffractometer (Beijing general instrument co., Ltd., XD-2X/M4600, CHN) in the range of 10-30° in
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steps of 2°. All the samples were freeze-dried and grinded into powder before used.
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2.5.7. Scanning electron microscopy (SEM) characterization
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The fracture surfaces of the hydrogel samples were observed by scanning electron microscope
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(SEM, EVO MA15, Zeiss, Germany). Before testing, all the samples were fractured with liquid
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nitrogen and freeze-dried immediately, and sputter-coated with a thin Au layer.
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3. Results and discussion
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3.1. Characterization of CNF and PVA-MA
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As shown in Fig. S2a, CNF dispersed well in water with the length ranging from one to several
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microns, which promoted the favorable dispersion of CNF into PVA-MA solution (Fig. S2b).
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Moreover, the insets of Fig. S2a and Fig. S2b depicted that the tyndall effect emerged by irradiating
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174
was dispersed well in aqueous solution or PVA-MA solution. The average particle size (267nm, Fig.
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S3) and surface with negative charge (Fig. S4) of CNF were tested by DLS. The functional carboxyl
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groups in the surface of CNF can effectively inhibit the aggregation of CNF, which can promote the
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sufficient interaction of CNF with PVA matrix. PVA-MA was firstly characterized by 1H NMR (Fig.
178
1). Compared to the spectrum of pristine PVA, new peaks at 6.0 and 5.6 ppm appeared which
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ascribed to the characteristic double bond of methacrylate group, demonstrating the methacrylate
180
group was successfully grafted onto the pendant hydroxyl groups of PVA. And the degree of
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substitution (DS) of PVA-MA was determined as 4% by calculating the integration of characteristic
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peak of methacrylate at 6.0, 5.6 ppm and the backbone of PVA at 3.8 ppm. FTIR was also used to
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confirm the effectiveness of methacrylate functionalization of PVA-MA (Fig. 2a). The increased
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Fig. 1 1H NMR spectra of PVA-MA and the pristine PVA ((DMSO)-d6).
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intensity of the bands at 1720 cm-1 and 1640 cm-1 in PVA-MA compared to the pristine PVA was
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attributed to the C=O and C=C stretching vibration, indicating the methacrylate groups were
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successfully grafted to the PVA chains.
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3.2. FTIR analysis of PVA-MA/CNF hydrogel The effectiveness of photocrosslinking reaction and CNF incorporation were firstly verified by
189
FTIR. As shown in Fig. 2a, the evident band in PVA-MA hydrogel and PVA-MA/CNF hydrogel
190
sample at 1640 cm-1 (νC=C) disappeared indicating the successful occurrence of photoinitiated
191
gelation. And the stretching vibration of hydroxyl group at 3367 cm-1 shifted to 3350 cm-1 because of
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the incorporation of CNF in PVA-MA/CNF hydrogel sample, indicating the occurrence of hydrogen
193
bond of CNF to the PVA chains. Furthermore, the transparency of PVA-MA/CNF hydrogel
194
decreased evidently compared with that of PVA-MA hydrogel under the same thickness (Fig. 2b, c).
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This phenomenon is not only attributed to the light scattering of CNF particles, the high aspect ratio
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of CNF promoting the entanglement of polymer chains and the hydrogen bond between CNF and
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PVA chains induced the increasement of physical junctions of hydrogel network also affected the
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transparency. This behavior strongly demonstrates the efficient interaction of CNF to the PVA
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matrix.
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Fig. 2 (a) FTIR spectra of the pristine PVA, PVA-MA, PVA-MA hydrogel, PVA-MA/CNF1 hydrogel and CNF; Photographs of PVA-MA and PVA-MA/CNF1 hydrogel in top side (b) and vertical side (c).
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3.3. Mechanical performance of the PVA-MA/CNF hydrogel The compressive strength of hydrogel is an important property for load-bearing application in
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cartilage repair or other tissue engineering scaffolds [16, 17, 37, 38]. Fig. 3a clearly shows that the
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incorporation of CNF obviously improves the compressive stress of PVA-MA hydrogels. And the
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PVA-MA/CNF 1 hydrogel exhibits the highest compressive fracture stress (490 kPa) with the
206
fracture strain of 73%, which is higher than that of previously reported PVA hydrogels (Table 2, 3).
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Remarkably, the PVA-MA/CNF 1 hydrogel was foldable and quickly recovered to its original state
208
without any rupture after compression, while the PVA-MA hydrogel was easy to rupture into small
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pieces under the same compressive condition, demonstrating the excellent flexibility and elasticity of
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CNF reinforced PVA-MA hydrogels (Fig. 3b, Supplementary Movie 1).
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Table 2. The fracture stress and strain results of PVA-MA/CNF hydrogels and PVA-MA hydrogel Samples
fracture
PVA-MA
PVA-MA/CNF 0.5
PVA-MA/CNF 1
PVA-MA/CNF 2
PVA-MA/CNF 4
hydrogel
hydrogel
hydrogel
hydrogel
hydrogel
140
315
490
358
273
73
70
74
fracture
70
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strain (%)
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Fig. 3 (a) Compressive stress-strain curves of PVA-MA hydrogel and PVA-MA/CNF hydrogel; (b) Photographs of the compression behavior of PVA-MA hydrogel and PVA-MA/CNF1 hydrogel under
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Except displaying a significant improvement of stress at fracture, the PVA-MA/CNF hydrogels
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also possess high toughness which confirmed by the loading-unloading compressive tests. These
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cycle curves were obtained immediately after one loading-unloading cycle. As shown in Fig. 4a, the
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stress is increasing with the applied strain from 40%-70%, and it can return to the original state after
217
unloading for each cycle.
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The value of corresponding stress at specific strain is also consistent with that in fracture
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compression test. Specifically, no breakage or even visible cracking was observed for PVA-
220
MA/CNF 1 hydrogel after 300 successive loading-unloading compressive cycles at a set strain of
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50%, and no evident decrease of stress was shown in the first 50 cycles (Fig. 4b). Even at a set strain
222
of 60%, this gel still could resist more than 100 successive loading-unloading compressive cycles,
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and the value of stress remained about the same in the first 10 cycles (Fig. 4c). This excellent
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recovery property of PVA-MA/CNF hydrogel can be explained by the efficient energy dissipating
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Fig. 4 Toughness mechanical property of PVA-MA/CNF1 hydrogel. (a) Compressive stress-strain curves with a varying maximum compression strain, the inset is amplification figure of the initial part; (b) Compressive stress-strain curves at 50% strain under loading-unloading cycles, the inset is curves for 1st-50th cycles; (c) Compressive 12 stress-strain curves at 60% strain under loadingunloading cycles, the inset is curves for 1st-10th cycles.
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mechanism. During the process of PVA-MA/CNF hydrogel deformation, the reversible physical
226
interactions between PVA- CNF chains or PVA, CNF chains can dissipate energy effectively,
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resulting to the high mechanical strength and toughness of hydrogel.
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3.4. Crystallinity of PVA-MA/CNF hydrogel Diffraction patterns of PVA-MA, PVA-MA/CNF hydrogels are shown in Fig. 5. The
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photocrosslinked PVA-MA hydrogel showed a diffraction peak at 2θ = 19.4o, being assigned to the
231
orthorhombic lattice structure of semicrystalline PVA [11]. And a sharp high peak at 2θ = 22.6o and
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two overlapped weaker diffraction peaks at 2θ = 14.8o, 16.6o appeared in CNF sample [39], which
233
was consistent with the diffraction peak of crystallographic form of cellulose I. With the
234
incorporation of CNF to PVA matrix from 0%wt-4%wt, the crystallinity of PVA-MA/CNF hydrogel
235
increases from 0%-1%, and shows highest for PVA-MA/CNF 1 hydrogel, then decreases from 1%-
236
4%, and the CNF diffraction peak appeared for PVA-MA/CNF 4 hydrogel. The above result
237
demonstrates the CNF incorporation content has an appropriate value for increasing the crystallinity
238
of the composited hydrogels. The more CNF in PVA matrix is prone to generate hydrogen bond
239
between CNF-CNF, and less physical interaction between CNF-PVA chains, sinificantly disturb the
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13 Fig. 5 The XRD patterns of PVA-MA hydrogel, PVA-MA/CNF hydrogel and CNF.
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crystallization of PVA-MA hydrogels, and exhibiting lower mechanical strength which is consistant
241
with the result of mechanical test.
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3.5. Morphological analysis of PVA-MA/CNF hydrogel In order to investigate the 3D network structure of PVA-MA/CNF hydrogel, the scanning electron
244
microscopy (SEM) was used to analyze the hydrogel samples which were treated by freeze-drying
245
technique. And the photocrosslinked PVA-MA hydrogel was as a control sample (Fig. 6a1, a2). As
246
shown in Fig. 6b-e, no significant aggregation of CNF in PVA-MA/CNF hydrogel samples is
247
detected, demonstrating the CNF dispersed well in PVA matrix for composite hydrogels. Among the
248
different PVA-MA/CNF hydrogel samples, PVA-MA/CNF 2 (Fig. 6d1, d2) and PVA-MA/CNF 4
249
(Fig. 6e1, e2) clearly show the disorganized structure because of the more CNF-CNF interaction, and
250
the PVA-MA/CNF 1 (Fig. 6c1, c2) shows the best uniform interconnected porous structure. The
251
homogeneous structure of PVA-MA/CNF 1 with tighter networks was believed to be responsible for
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the excellent mechanical properties, which corresponds well with the result of FTIR, compressive
253
mechanical test and XRD. Furthermore, the interconnected holes are beneficial for the biological
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Fig. 6 Scanning electron microscopy images of PVA-MA hydrogel (a1, a2), PVA-MA/CNF0.5 hydrogel (b1, b2), PVA-MA/CNF1 hydrogel (c1, c2), PVA-MA/CNF2 hydrogel (d1, d2), and PVA-MA/CNF4 hydrogel (e1, e2), the scale bar is 10 µm (top) and 3 µm (bottom).
254
property of hydrogels, which promote cells grow and migrate or make nutrients or growth factors
255
into the material inside.
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4. Conclusions 14
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with CNF reinforcement. Except avoiding using the small molecule chemical crosslinking agent and
259
time-consuming freeze-thaw process, a much higher mechanical strength PVA hydrogel was
260
successfully prepared by optimizing the CNF loading content, and the compressive fracture stress
261
reaches 490 kPa which is higher than that of previously reported nanocellulose reinforced PVA
262
hydrogels. Specifically, this kind of PVA-MA/CNF hydrogels exhibit excellent toughness, which can
263
resist hundreds of successive loading-unloading compressions. And the mechanism of CNF
264
reinforcement was systematically studied. The biocompatibility of the synthetic tough PVA hydrogel
265
and its degradation products will be done in future work. This fabricated tough PVA hydrogels
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greatly advance their potential to be applied in tissue engineering scaffold and biomedical field.
267
Acknowledgements
268
This work was supported by National Natural Science Foundation of China [31741022]; Natural
269
Science Foundation of Guangdong Province [2018A030310146].
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Table 3. The fracture stress and strain results of different kinds of PVA hydrogels PVA weight
filler / filler weight
time (h)
weight (g/mol)
percent (%wt)
percent (%wt)
Compressive fracture stress (kPa) and fracture strain (%)
photocrosslinking
0.1
94,899
10 (PVA-MA)
CNF, 1
490, 73
[19]
1 freeze-thaw cycle
14
89,000-98,000
4
CNF/LCNF, 2
≈138, 60
[18]
3-5 freeze-thaw cycles
≈104
89,000-98,000
10
CNC, 1
37, 50
[20]
5 freeze-thaw cycles
50
88,000
10
graphite oxide, 0.025
102, 53
2
N/A
1.5
Acrylamide, 8
16, 75
chemical crosslinking with borax chemical crosslinking with borax
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with glutaraldehyde
0.08
89,000-98,000
4
CNC, 1
60, 53
1
146,000-186,000
2
CNC/CNF, 1
71, 95
5
CNC, 40
33, N/A
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acrylamide
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process
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gelation
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ACCEPTED MANUSCRIPT Highlights: A tough polyvinyl alcohol hydrogel was fabricated facilely by using photocrosslinked polymerization and cellulose nanofibrils (CNF) reinforcement.
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The mechanism of CNF reinforcement was displayed explicitly. The fracture compressive stress of this PVA-MA/CNF hydrogel reaches 490 kPa which is higher than that of previously reported nanocellulose reinforced PVA
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hydrogels.
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